The desirability of forming bipolar and field effect transistors on the same monolithic semiconductor substrate is well known. Many proposals have been made in the prior art for accomplishing that purpose. Generally, the previously proposed techniques involve either compromising optimized processes (for making the bipolar and field effect transistors separately) in an effort to maintain desirable device performance characteristics or compromising desirable performance characteristics (of either or both types of transistors) in an effort to maintain a simplified process. One of the major obstacles standing in the way of prior art success is the fact that the impurity concentration levels associated with bipolar transistor substrates are an order of magnitude different from those associated with field effect transistors. Accordingly, there is a need for a technique for the fabrication of bipolar and field effect transistors on the same monolithic substrate permitting substantially independent impurity concentration control while using process steps previously optimized for the fabrication of bipolar and field effect transistors when made separately.